I do not think 343i should work on and release Halo 3 Anniversary before Halo 6. The reason is that due to 343i working on Halo MCC and Halo 5 and rushing, two games were launched incomplete and broken.
I do not want the same thing to happen to Halo 6. I think Halo 6 could be a great game, possibly the next Halo 3 for the community, so I want them to put all their focus on Halo 6.
I do want Halo 3 Anniversary eventually, but I want them to take their time with Halo 6. I think after Halo 6 is released, than I would want them to start development on H3A, but again take time with it and release it as a complete game.